I don't think you did. "Should be" implies a norm that the manufacturer are obliged to follow. There is no such thing when it comes to the DxOMark "measured ISOs". The only norm the manufacturers are obliged to follow concern OOC jpegs, not RAWs.

Anders, this type of argument about comparable ISO has been pervading the forum for ages, and it would be really useful for everyone to come to some agreement about what it means.

I couldn't agree more.

I doubt my questions will solve the battle but I am going to give it a try.

It's a pretty good try.

1. What do you think is a better way to compare these cameras, by using the same ISO or by using the same aperture and shutter speed? I use the word "better" in this question but you could interpret it to mean "fairer" for the purpose of establishing which camera produces the best image quality in the same conditions. I also wish to note that the comparison between the GH3 and the EM5 are a rare opportunity to remove the lens from the equation because both cameras are tested with the same 50mm F2 lens on the DPReview tests, so my question in the first instance relates specifically to a comparison between the GH3 and the EM5, but you could add comments about a more general comparison too.

First, what I think you really mean here is not just the same aperture and shutter speed but the same exposure, which can be obtained, for example by using the same f-stop, the same shutter speed, and, last but not least, the same light conditions. Second, if that's what you mean, the question is not whether we should compare cameras at the same camera ISO or at the same exposure. Rather, I would say that a fair way to compare cameras is to shoot them at the same camera ISO and the same exposure.

Furthermore, that's for the most part exactly what DPR is doing, although a lot of people mistakenly think that such is not the case. These people look at the samples from two cameras and note (for example) that the shutter speed used was longer for one than for the other while the f-stop remained the same. They then jump to the mistaken conclusion that the exposure used was not the same. This conclusion rests on the assumption that the light conditions were identical and DPR has made it clear that this assumption is incorrect. The light level in the studio where they shoot the studio scene samples is not controlled and they make no particular effort to keep it constant. Consequently, variations in shutter speed are as a rule simply a way of compensating for variations in the studio light level.

What I say in the previous paragraph rests on the presumption that both cameras have passed the DPR ISO accuracy test reported in the "noise and noise reduction" section of each review without any discrepancy. The large majority of cameras do so but there are exceptions. The E-M5 is one of them. In this case, DPR reports that there is a discrepancy of 1/3 EV in the direction of "too dark". This would ordinarily mean that the E-M5 enjoys the extra benefit of 1/3 EV more exposure when the studio scene samples are subsequently shot, and that you would have to keep that extra advantage in mind when comparing the E-M5 samples with those of other cameras.

Now there are ways to check whether two cameras actually got the same exposure for the studio scene samples or not (although, as already pointed out, it is not possible by simply checking the f-stop and shutter speed used). For a variety of reasons, I have performed that check for the E-M5 in comparison with each of three other cameras (the GH2, the D800, and the GH3) and in no case found any trace of the 1/3 EV extra benefit that DPR's ISO accuracy report gives us reason to expect. So in spite of what DPR says in the "noise and noise reduction" section of the E-M5 review, the E-M5 studio scene samples are fully comparable to those of other cameras.

For those interested in the methods used to perform this check, it is exemplified here with regard to the E-M5 versus the GH3:

2. Do you think there is a "normal" response (with required aperture and shutter settings) from cameras to achieve a given ISO? If you took all the current manufactured cameras (or a suitable subset) and produced a table of ISO to aperture and shutter speed for one specific condition of lighting and scene, could we establish a normal response with which to compare all cameras?

Thanks.

(apologies in advance for possible slow response).

I have to rush away now, so I'll have to return to your second question at a later time.